Implementing diesel::result::DatabaseErrorInformation for Infallible

[LucaCappelletti94]

Backend

PostgreSQL, MySQL, SQLite, Others

What do you want to add to diesel?

At the moment I have an error type of the form:

pub enum Error<E> {
    Validation(E),
    OtherThing { table: String, column: String },
}

I implement DatabaseErrorInformation for Error<E> whenever E: DatabaseErrorInformation, by delegating to the inner error for the Validation variant and providing static information for the others.

In some use cases I want to express that validation is always correct, i.e. Error<Infallible>. However, this currently does not type-check because std::convert::Infallible does not implement DatabaseErrorInformation.

From a semantic point of view, Infallible is an uninhabited type and can never be instantiated, so any implementation of DatabaseErrorInformation for it would be unreachable at runtime. Still, due to the orphan rules, it is impossible to provide such an implementation downstream.

This would allow generic error types to use Infallible as a marker for “this branch cannot occur”, without requiring ad-hoc wrapper types or loss of type information.

Implementation notes

Would it be acceptable for Diesel to provide a built-in implementation like:

impl DatabaseErrorInformation for std::convert::Infallible {
    fn message(&self) -> &str {
        match *self {}
    }

    fn details(&self) -> Option<&str> {
        match *self {}
    }

    fn hint(&self) -> Option<&str> {
        match *self {}
    }

    fn table_name(&self) -> Option<&str> {
        match *self {}
    }

    fn column_name(&self) -> Option<&str> {
        match *self {}
    }

    fn constraint_name(&self) -> Option<&str> {
        match *self {}
    }
}

This is statically unreachable code: there is no panic path and no runtime behavior, just a type-level proof that the methods cannot be called.

Additional details

No response

Checklist

• I have already looked over the issue tracker and the discussion forum for similar possible closed issues.

[LucaCappelletti94]

Closing as completed with PR #4952